In the longitudinal study of affectively ill and well parents, parent- child relationships have been evaluated in early childhood, late childhood and early adolescence. The relationship of the two siblings in the family is also of interest in assessing the socioemotional development of children at risk for affective illness. observations of sibling interaction were made at the beginning of the study, when the younger sibling was between 1 1/2 and 3 1/2 years of age and the older sibling was between 5 and 8 years of age, and again at two later developmental periods. The objective of the study is to examine patterns of interaction in the areas of affect, communication, and play behavior in these sibling pairs across the three developmental periods in terms of parental psychopathology, child problems, and gender composition of the dyad. In addition, children's representations of the sibling relationship will be considered.